1. Field of the Invention
The present invention relates to a soft magnetic material, particularly a Mn--Zn ferrite suitable for low loss material for a transformer for switching power supply, a rotary transformer and deflection yoke, electronic parts such as for various kind of inductance elements and impedance elements for countermeasure against EMI, or for electromagnetic wave absorbers.
2. Description of the Related Art
The representative oxide magnetic material having a soft magnetism includes Mn--Zn ferrite. Conventionally, this Mn--Zn ferrite generally has a basic component composition containing more than 50 mol %, 52 to 55 mol % on the average, of Fe.sub.2 O.sub.3, 10 to 24 mol % of ZnO and the remainder being MnO. In general, the Mn--Zn ferrite is produced by mixing each of raw material powders of Fe.sub.2 O.sub.3, ZnO and MnO in predetermined proportions, forming a predetermined shape through the steps of calcination, milling, component adjustment, granulation and pressing, and then conducting a sintering treatment by maintaining the green compact at 1,200 to 1,400.degree. C. for 3 to 4 hours in reducing atmosphere which suppresses an oxygen concentration by flowing nitrogen. The reasons for sintering in reducing atmosphere are as follows. If the green compact is sintered in air when Fe.sub.2 O.sub.3 contains more than 50 mol %, densification does not proceed sufficiently, so that preferable soft magnetism is not obtained. Further, Fe.sup.2+ formed by reduction of Fe.sup.3+ has a positive crystal magnetic anisotropy, and has an effect to erase a negative crystal magnetic anisotropy of Fe.sup.3+, thereby improving a soft magnetism. However, if sintering is conducted in an air, such a reductive reaction cannot be expected.
Where a Mn--Zn ferrite is used as a magnetic core material, eddy current flows as a frequency region used increases, and loss by the eddy current increases accordingly. Therefore, in order to increase the upper limit of the frequency which can be used for a magnetic core material, it is necessary to make its electrical resistance large as much as possible. However, the electrical resistance in the above-described general Mn--Zn ferrite is a value smaller than 1 .OMEGA.m due to enjoyment of electrons between the above-described Fe.sup.2+ and Fe.sup.2+ (interionic), and the frequency which can be used is limited up to approximately several hundreds of kHz. Initial permeability is considerably decreased in the frequency region exceeding 1 MHz, and properties as the soft magnetic material are completely lost.
In some instances, however, such a countermeasure is taken that in order to increase the electrical resistance of Mn--Zn ferrite, CaO, SiO.sub.2 and the like are added as additives to the above-described main components to make the resistance of grain boundary high, and also sintering is conducted at a low temperature of approximately 1,200.degree. C. to reduce the grain size up to approximately 5 .mu.m, thereby increasing the proportion of the grain boundary. However, it is difficult to obtain electrical resistance exceeding 1 .OMEGA.m even with such a countermeasure, and a fundamental solution is not yet attained.